Method of sum-frequency conversion and frequency converter with optical active rotator

ABSTRACT

A method for sum-frequency conversion of coherent radiation includes generating two linearly polarized waves which collinearly propagate in respective first and second non-coinciding planes at different first and second frequencies f 1 π/f 2 , respectively. The waves are further guided through an active crystal which rotates the waves at different angles Ψ 1  and Ψ 2  determined as 
       Ψ 1 =ρ( f   1 )· L  
 
       Ψ 2 =ρ( f   2 )· L,  
 
     where L is a length of the active crystal, and ρ(f 1 ) and ρ(f 2 ) specific rotations of respective frequencies f 1 π/f 2 . Finally the rotated waves are incident on a non-linear crystal configured to sum frequency the wave to generate a third harmonic.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/589,085 filed with the U.S. Patent Office on Jan. 20, 2012.

BACKGROUND OF THE DISCLOSURE

Currently, there is a growing demand for high power ultraviolet (UV)pulsed lasers for various industrial applications such as LED scribing,chip dicing, via-hole drilling, plastics marking and others. Incomparison with more common IR lasers, UV lasers have an advantage ofhigher linear and nonlinear absorption of the UV light by some materialsand their possibility to achieve smaller focus spots. The majority ofcommercially available pulsed UV lasers are diode pumped solid stateNd:YVO₄, Nd:YAG, or Nd:YLF lasers or Yb-doped fiber master oscillatorpower amplifier (MOPA) lasers operating near 1 μm wavelength with inter-or intra-cavity frequency tripling or quadrupling.

The conventional way of third harmonic generation (THG) employed in mostof the UV lasers operating near 0.35 μm consists of a two stage process:second harmonic generation (SHG) in a type-I phase-matched nonlinearoptical crystal and sum frequency generation of the fundamental andsecond harmonics in a type-II phase-matched crystal. Usually LiB₃O₅(LBO) crystals are used for both processes due to their high damagethreshold, high nonlinearity, low absorption in visible and UV ranges,and high crystal growth yield. The popularity of the described schemecan be explained by its ease of implementation: in the output of thefirst nonlinear crystal the fundamental and double frequency waves arepolarized in the orthogonal planes which is exactly what is required fortype-II phasematching condition in the second nonlinear crystal. Becauseof that there is no wave manipulation needed between the nonlinearcrystals except for focusing of the waves.

The alternative way of THG is to use type-I phase-matched crystals forboth processes. Compared to the above-discussed technique, this one isassociated with higher conversion efficiency of sum-frequency generationunder type-I phasematching condition. For example, the total conversionefficiency is about 2.2 times higher in the type-I phase-matched LBOcrystal than the one in the type-II crystal at 355 nm wavelength at 100°C. In addition, there is no spatial walk-off of the fundamental andsecond harmonic waves in the type-I phase matching scheme, which removesthe crystal length limitation present in type-II phase-matching. Thereis, however, non-zero spatial walk-off of the third harmonic wave withrespect to the fundamental and the second harmonic waves. This effectleads to some ellipticity of the 355 nm output wave, which is consideredto be a minor problem and could be compensated, for example, by ananamorphous prism arrangement. Thus due to higher nonlinearity and theabsence of crystal length limitation, THG in a type-I LBO crystal issignificantly more efficient. This is especially important for deviceswith low IR pump peak powers, such as fiber lasers. Higher efficiencyalso decreases a rate of crystal degradation by relaxing the focusingconditions in the THG crystal.

The peculiarity of the type-I phase-matching scheme, however, is thefact that both fundamental and double frequency waves must be polarizedin the same plane. That means that a polarization control element isrequired after the first nonlinear crystal. Usually a wave plate isinserted between the nonlinear crystals for that purpose. This waveplate should simultaneously provide a half wavelength phase shift to thefundamental wave and a whole wavelength phase shift to the secondharmonic wave. If the phase axis of such a wave plate is oriented at 45°with respect to the polarization plane of the fundamental wave, the waveplate will flip the fundamental wave polarization by 90 degrees, whilethe polarization of the second harmonic wave will remain unchanged. As aresult, the polarizations of the fundamental and second harmonic wavesbecome collinear.

As one of ordinary skill in the geometric optics is well aware, the useof the wave plate may be problematic. One of the known problems includesthe dependence of the phase shift from high temperatures which typicallyleads to the third harmonics power instability. Another problem stemsfrom resonant wavelength dependence that requires very high precisionmanufacturing of the wave plate, which, in turn, drives the cost up.Still another problem relates to the requirement of precise angularadjustment around the wave propagation axis which complicates the waveplate installation.

As also known to one of ordinary skill, because of the dispersion of theabsolute value of the ordinary and extraordinary indexes of refraction|n_(o)−n_(e)|, it is impossible to make a single wavelength phase shiftwave plate which works at both fundamental and second harmonicwavelengths. Thus the required wave plate will have a phase shift ofconsiderable number of integer wavelengths (N) and in turn will have Ntimes higher thermal dependence. For example, in order to get anacceptable angular mismatch between the fundamental and second harmonicwaves polarization planes of Δδ≈0.3° one has to use a 91th order 5.55 mmthick quartz wave plate. A simple calculation shows that the temperaturechange from 0 to 50° C., which is a typical industrial applicationsoperation temperature range, leads to a prohibitively high change of thepolarization planes angular mismatch of Δδ=110°.

Based on the foregoing, a need therefore exists for a frequencyconverter configured with an optical active rotator avoiding problemsassociated with the above-discussed wave-plate.

A further need exists for a method of sum-frequency conversion utilizingthe disclosed active rotator.

SUMMARY OF THE DISCLOSURE

These needs are met by the disclosed structure operative to generate athird or higher harmonic of fundamental frequency by utilizing an activecrystal which is configured to align polarization planes of two wavesupstream from a type-I phase matched crystal.

In accordance with one aspect of the disclosure, the disclosed methodfor sum-frequency conversion of coherent radiation includes propagatingtwo collinear waves in respective first and second non-coinciding planesat different first and second frequencies f₁π/f₂, respectively. Thewaves are guided through an active crystal so as to be simultaneouslyrotated at different angles Ψ₁ and Ψ₂. Then the waves are incident on anonlinear crystal which is configured to realize the sum-frequencyconversion of the two frequencies into a third frequency. The angles atwhich the waves are rotated are determined as Ψ₁=ρ(f₁)·L and Ψ₂=ρ(f₂)·L,where L is a length of the active crystal, and ρ(f₁) and ρ(f₂) specificrotations of respective frequencies f₁π/f₂.

In a further aspect of the disclosure, the disclosed frequency converteris configured with an upstream optically active crystal operative tosimultaneously rotate two collinear waves at different angles Ψ₁ and Ψ₂,which propagate in different planes and at different frequencies. Thedisclosed converter further includes a downstream nonlinear crystalrealizing sum-frequency conversion of two frequencies (f1) and (f2) ofrespective collinear waves into a third frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages will become more readilyapparent from the following specific description accompanied bydrawings, in which;

FIG. 1 is an optical schematic of the disclosed frequency converteroperative to generate a third harmonic.

FIG. 1A is an exemplary optical scheme of the frequency converter ofFIG. 1.

SPECIFIC DESCRIPTION

Reference will now be made in detail to the disclosed energy absorber,high power fiber laser system incorporating the absorber and a methodfor manufacturing the latter. Wherever possible, same or similarreference numerals are used in the drawings and the description to referto the same or like parts or steps. The drawings are in simplified formand are far from precise scale. For purposes of convenience and clarityonly, directional terms may be used with respect to the plane of thedrawing sheets and not be construed to limit the scope. Unlessspecifically noted, it is intended that the words and phrases in thespecification and claims be given the ordinary and accustomed meaning tothose of ordinary skill in the fiber laser arts.

Referring to FIG. 1, a converter 100 illustrates a general concept ofthe present disclosure. As one of ordinary skill knows, the frequencytripling is the phenomenon in accordance with which an input beamgenerates an output beam with a frequency which is three times that ofthe fundamental frequency. The process of frequency tripling is usuallyrealized as a multi-step process including first frequency doubling ofthe input beam and subsequent sum frequency generation of both waves.

Accordingly, a source 101 is operative to emit a linearly polarizedsingle mode (“SM”) beam propagating along a path at a fundamentalfrequency, which is the frequency of the given laser. The SM beam isincident on an upstream non-linear crystal 111 which is the I-typecrystal converting the fundamental frequency into its second harmonic.As a result two waves at the fundamental and second harmonicfrequencies, respectively, propagate further along the path in twopolarization planes which are orthogonal to one another.

As one of ordinary skill knows, to obtain the third harmonic, the planesof the fundamental and second harmonic frequencies should coplanar.Since the planes of the waves downstream from upstream non-linearcrystal 111 are orthogonal, both planes, which are rotatable atdifferent velocities, should be rotated at different angles in order tocoincide with one another.

A rotator 112 located along the path of the non-planar waves isconfigured to realize such as rotation. In contrast to the known priorart, rotator 112 is configured as an active linear crystal, i.e., thecrystal capable of rotating a plane of polarization. More particularly,active rotator 112 is configured as an active optical crystal (e.g.crystallized quartz, LiIO₃, TeO₂, etc.) Specific optical activity(polarization rotation angle per unit length) r in crystal 112 has astrong wavelength dependence (it is monotonically decreases in the nearIR-visible range). As a result, if both waves at fundamental and secondharmonic frequency, respectively, are propagating through an opticallyactive material in the optical axis direction, their polarization planeswill rotate with different speeds and will exactly coincide at a certainposition. The minimal length L of the optically active crystal to getthe fundamental and second harmonic beams polarizations aligned may becalculated by this formula:

$\begin{matrix}{{L = \frac{90{^\circ}}{\rho_{SH} - \rho_{FH}}},} & (1)\end{matrix}$

Where p_(FH) and p_(SH) are specific optical activities at fundamentaland second harmonic wavelengths respectively.

Returning to FIG. 1, two coplanar waves are further incident on a I-typenonlinear crystal 113 configured to sum frequency the waves so as togenerate a third harmonic propagating. The I-type non-linear crystals111 and 113 may be grown from the same or different materials. Forexample, these crystals each include Lithium Triborate (“LBO”). The SMoutput beam 202 is emitted at the triple frequency.

Turning now to FIG. 1A, a specific embodiment of FIG. 1 includes an Ybfiber laser used as a pump source. The emitted SM is emitted at about1064 nm and further propagates through a collimator and a focusing lens1. The focused beam is incident on a first I-type non-linear crystalsecond harmonic generator SHG LBO doubling the incident beam to outputboth waves at 1064 nm and 535 nm propagating in respective orthogonalplanes. Thereafter, the waves are rotated in the optical rotator at thedesired angles to coincide with one another. Upon further focusing inlens 2, a downstream third harmonic generation crystal THG LBO generatesa wave at 355 nm. Three waves including respective fundamental, secondand third harmonic frequencies are further incident on a dichroic mirrorwhich taps the desired 355 nm wave finally measure in a power meter.

The following table exemplifies the required crystal length L,polarization rotation angle φ_(FH) and specific optical activity ρ_(FH),ρ_(SH) for SiO₂ (quartz) and TeO₂ crystals at 1064 and 532 nm,respectively.

r_(PH) (deg/mm), r_(SH) (deg/mm), Material L (mm) f_(FH) (deg) 1064 nm532 nm SiO₂ 4.39 27.7 6.3 26.8 TeO₂ 0.77 19.9 25.9 143.4

As disclosed above, the scheme of FIGS. 1 and 1A is based on I-typenonlinear crystals which can be considered inly in the context of twoorthogonally-polarized waves. However, the waves incident on activecrystal 112 may be oriented at an arbitrary angle Ψ different from theright angle. In this case, the conversion may be realized on either theI-type crystal or the II-type crystal. In the sum-conversion occurs inthe I-type crystal, then the length L of active crystal 112 isdetermined as follows:

L=Ψ/Δρ  (2),

where Δρ is difference between specific rotations of the waves atrespective fundamental and arbitrary higher harmonic frequencies. Theabove is given for one of rotational directions of the planes. In theother direction opposite to the one direction, the length can bedetermined as

L=(180°−Ψ)/Δρ  (3)

If the sum-frequency occurs in the II-type crystal, in one of theopposite rotational directions, the length L of the active crystal isdetermined as

L=(90°+Ψ)/Δρ  (4)

In the opposite direction, the length of the active crystal can bedetermined as:

L=(90°−Ψ)/Δρ  (5)

Note that above-disclosed scheme can be utilized for sum-frequency ofother than the fundamental frequency and a higher frequency. Forexample, the scheme operates for fourth and fifth harmonics of thefundamental frequency.

Although shown and described is what is believed to be the mostpractical and preferred embodiments, it is apparent that departures fromspecific designs and methods described and shown will suggest themselvesto those skilled in the art and may be used without departing from thespirit and scope of the invention. The present invention is notrestricted to the particular constructions described and illustrated,but should be construed to cohere with all modifications that may fallwithin the scope of the appended claims.

1. A method for sum-frequency conversion of coherent radiation, comprising: guiding a single mode linearly polarized first wave at a frequency f₁, thereby generating a second linearly polarized wave at a frequency f₂ different from frequency f₁, the first and second waves being collinear and propagating in respective first and second non-coinciding polarization planes; guiding the two waves through an active crystal, therefore simultaneously rotating the two waves at different angles; and thereafter guiding the two waves through a nonlinear crystal, therefore realizing sum-frequency conversion of the first and second frequencies into a third frequency, wherein a length L of the active crystal is determined as L=(n°±Ψ)/Δρ, where Δρ is difference between specific rotations of the waves at respective first and second frequencies f₁π/f₂.
 2. The method of claim 1, wherein the first and second waves propagate in respective first and second orthogonal planes, the non-linear crystal being a typed phase matched crystal, and n°±Ψ being equal to 90°
 3. The method of claim 2 further comprising determining the length L of the active crystal as $L = {\frac{90{^\circ}}{\rho_{SH} - \rho_{FH}}.}$
 4. The method of claim 1, wherein the non-linear crystal is configured to be a type-II phase matched crystal.
 5. The method of claim 4 further comprising determining the length L of the active crystal as L=(90°+Ψ)/Δρ provided the first and second polarization planes rotate in one of opposite directions.
 6. The method of claim 1, wherein the frequency f₁ is a fundamental frequency and the frequency f2 is a second harmonic of the fundamental frequency.
 7. The method of claim 1, wherein the frequency f₁ is either a fundamental frequency or a higher frequency.
 8. The method of claim 1, wherein the non-linear crystal includes Lithium Triborate (LBO, LiB3O5).
 9. A frequency converter, comprising: an upstream optically active crystal operative to simultaneously rotate two collinear waves at different angles Ψ₁ and Ψ₂; and a downstream nonlinear crystal realizing sum-frequency conversion of two frequencies (f1) and (f2) of respective collinear waves into a third frequency.
 10. The frequency converter of claim 9, wherein the first and second waves propagate in respective first and second orthogonal planes, the non-linear crystal being a type-I phase matched crystal, and n°±Ψ being equal to 90°
 11. The frequency converter of claim 10, wherein the active crystal is configured with a length L determined as $L = {\frac{90{^\circ}}{\rho_{SH} - \rho_{FH}}.}$
 12. The frequency converter of claim 10, wherein the non-linear crystal is configured to be a type-II phase matched crystal.
 13. The frequency converter of claim 12, wherein the active crystal is configured with a length L determined as L=(90°±Ψ)/Δρ depending n a rotational direction of the first and second polarization planes.
 14. The frequency converter of claim 10, wherein the active crystal comprises quartz, LiIO₃, TeOi.
 15. The frequency converter of claim 1, wherein the non-linear crystal includes Lithium Triborate (LBO, LiB3O5). 